Position indicators using magnetic field detectors is known and an example of such device is described in U.S. Pat. No. 5,434,500 issued on Jul. 18, 1995. That device uses a magnetic field generator and a receiving or detecting unit which is able to detect the magnetic field on an opposed side of a barrier. The magnetic field generator is placed on a first side of the barrier at a spot to be located and creates a magnetic field at that spot. The receiving unit is composed of four detectors which measure the relative strength of the magnetic field at each detector and this allows the position of the source to be determined. The strength of the field is indicated by light emitting diodes. When the strength of the field is equal at all for points the receiving unit is centered approximately on that point on the opposite side of the barrier from the sending unit. The receiving unit has a center hole therein to introduce a pencil to mark a position on the surface of the barrier. Such device has been found adequate to locate the position of the center of the receiving unit from the other side of the barrier but it is fairly bulky and expensive to produce. When the wall is thick and dense, the device may not perform adequately.
There is also a need for such devices to detect the thickness of a barrier whether it is a concrete wall barrier or a wooden wall structure as we commonly find in home construction. There is also a need to provide a device which can be carried in the pocket of a user person and which is inexpensive to produce and easy to use and which provides accurate measurements of the thickness of a barrier as well as the location of a magnet generating a magnetic field attach to an opposed surface of that barrier.